1. Field of the Invention
The present invention relates to an optical fiber-type wave-divider-multiplexer for dividing and multiplexing light of a plurality of wavelengths, and more particularly to an optical fiber-type wave-divider-multiplexer whose wavelength isolation performance is improved.
2. Description of Conventional Art
As a wave-divider-multiplexer (WDM) for dividing and multiplexing the light of different wavelengths among a plurality of optical fibers, an optical fiber-type wave-divider-multiplexer using optical fibers is known. Referring to FIG. 3, a description will be given of an example of a configuration of an optical fiber line using the optical fiber-type WDM which makes use of two wavelengths. In the drawing, reference numerals 11 and 12 denote optical fiber-type WDMs; numeral 13 denotes an optical communication line; reference characters a and b denote coupled points; T.sub.x1 and T.sub.x2 denote light transmitters; and R.sub.x1 and R.sub.x2 denote light receivers. Ends of optical fibers from the optical fiber-type WDM 11 and 12 are coupled to an optical fiber of the optical communication line 13 at the points a and b.
FIG. 3A shows a configuration of one-way communication. In this example, optical signals from the light transmitter T.sub.x1 for transmitting a 1300 nm wavelength and from the light transmitter T.sub.x2 for transmitting a 1550 nm wavelength are combined into one optical signal by the optical fiber-type WDM 11 so as to be transmitted via one communication line 13. On the receiving side, the light receiver R.sub.x1 for receiving the 1300 nm wavelength and the light receiver R.sub.x2 for receiving the 1550 nm wavelength are coupled to the optical fiber-type WDM 12. The optical signal transmitted from the optical communication line 13 is divided into the respective wavelengths by the optical fiber-type WDM 12, and are received by the respective light receivers R.sub.x1 and R.sub.x2. In this example, one optical communication line can be used as one-way communication line. In this case, the optical fiber-type WDM 11 is used as a multiplexer, while the optical fiber-type WDM 12 is used as a divider.
FIG. 3B shows an optical fiber line for two-way communication. In this example, transmission and reception between the respective terminals can be effected by using the 1300 nm wavelength and the 1550 nm wavelength. That is, one terminal has the light transmitter T.sub.x1 for transmitting the 1300 nm wavelength and the light receiver R.sub.x2 for receiving the 1550 nm wavelength. In contrast, the other terminal has the light transmitter T.sub.x2 for transmitting the 1550 nm wavelength and the light receiver R.sub.x1 for receiving the 1300 nm wavelength. Accordingly, communication from one terminal to another can be effected by the optical fiber-type WDMs 11 and 12 by the 1300 nm wavelength, while communication from the other terminal to one terminal can be effected by the optical fiber-type WDMs 12 and 11 by the 1550 nm wavelength. In this example, transmission and reception can be mutually effected by one optical communication line. In this case, it can be said that the optical fiber-type WDMs 11 and 12 are used as directional couplers rather than as wave-divider-multiplexers.
An outline of the above-described optical fiber-type wave-divider-multiplexer (WDM) is shown in FIG. 6. In the drawing, reference numerals 1a and 1b denote optical fiber cladding portions; 2a and 2b denote glass portions; 3 denotes an optically coupled portion; 4 denotes a fixing member; and 5 denotes an adhesive. A melt drawing method is known as a most popular method of manufacturing the optical fiber-type WDM such as the illustrated one. This method is described in, for instance, NEW GLASS, vol. 6, No. 1, published in 1991, pp. 48-59. In this method, the cladding 1a and 1b of a plurality of optical fibers is partially removed to expose the glass portions 2a and 2b, and the exposed portions are then heated by a burner or the like so as to be integrated while being brought into close contact with each other. This integrated portion is further heated and is drawn, thereby forming the optically coupled portion 3. At that time, the light made incident upon one end of the optical fiber is measured at the other end thereof to detect a multiplexing and dividing characteristic (branching ratio), and drawing is stopped when a desired characteristic is obtained. Then, finally, the optical fibers are fixed to the fixing member 4 by the adhesive 5, thereby obtaining the optical fiber-type WDM such as the one shown in FIG. 6. An optical fiber-type WDM produced by this process is also known as a fiber fusion type WDM.
The optical fiber-type WDM thus manufactured has a function in which when the light of wavelengths .lambda..sub.1 and .lambda..sub.2 is simultaneously made incident upon an incident port, the light of the wavelength .lambda..sub.1 is made emergent from an emergent port 1 and the light of the wavelength .lambda..sub.2 is made emergent from an emergent port 2, thereby dividing the light into the light of the wavelengths .lambda..sub.1 and .lambda..sub.2. It should be noted that the optical fiber-type WDM has a multiplexing function in which the light of the wavelengths .lambda..sub.1 and .lambda..sub.2 made incident upon the emergent ports 1 and 2 and the illustrated port is respectively combined and is made emergent from the incident port and the illustrated port.
A description will be given of the wavelength dividing characteristic by assuming that the intensity of light made incident upon the incident port is P.sub.0, the intensity of light made emergent from the emergent port 1 is P.sub.1, and the intensity of light made emergent from the emergent port 2 is P.sub.2. FIG. 7 is a characteristic diagram of the conventional optical fiber-type WDM described in connection with FIG. 6. In the drawing, the broken line indicates P1-P0, and the solid line indicates P2-P0. As can be seen from this diagram, P1 is large and P2 is small at the wavelength .lambda..sub.1 ; namely, the diagram shows that most of the light of the wavelength .lambda..sub.1 is outputted to the emergent port 1. With respect to the wavelength .lambda..sub.2 as well, it can also be seen that the light is outputted to the emergent port 2.
The wavelength isolation which is the wavelength dividing capability is called separation, and it means that the greater the separation the greater the wavelength dividing capability. With reference to FIG. 7, for instance, the difference between the broken line and the solid line at the wavelength .lambda..sub.1 shows the magnitude of the separation. As for the magnitude of the separation, 20 dB or more is desired in applications to communications, sensors, and the like. With the conventional optical fiber-type WDM, however, there is a problem in that the separation becomes small if an attempt is made to reduce a wavelength interval .DELTA..lambda.(=.lambda..sub.1 -.lambda..sub.2) to about 100 nm or less, or if an attempt is made to alleviate the wavelength characteristic of the light transmitter by broadening the allowable wavelength width of .lambda..sub.1 and .lambda..sub.2. Hence, with the conventional optical fiber-type WDM, it was possible to obtain only the separation of 15 dB or thereabout. For this reason, there has been a problem in that the dynamic range of the transmitted light is small, and the communication distance using optical fibers cannot be made long, and that it is necessary to employ expensive high-output lasers and very high-sensitivity light receivers.
In addition, if the light of a wavelength in the other channel leaks, that constitutes noise and results in a deteriorated S/N ratio, presenting a problem in image communication such as TV. In particular, if an attempt is made to ensure wavelength separation with a practical wavelength width which takes into consideration the wavelength width of a light source, a lower value than a single-wavelength assurance is obtained.
Factors which impair the dynamic range include:
(1) a loss due to the optical fiber (dependent on the distance of optical fiber communication),
(2) a loss due to an intervening optical fiber-type wave-divider-multiplexer or coupling portion,
(3) a decline in the SN ratio (depending on a communication system), and
(4) others, including the bending loss of the optical fiber and the light source power.
Accordingly, in the optical fiber-type WDM, raise of the separation is an important issue. It is conceivable to provide an arrangement in which an optical fiber with an optical filter is connected to a fiber for photometrically measuring a divided wavelength so as to impart a wavelength selecting characteristic to the incident side or emergent side of the optical fiber-type WDM, thereby to increase the separation and improve the S/N ratio. However, the insertion of the optical fiber with an optical filter into the optical wiring of the optical fiber-type WDM means that a member having a large volume is inserted into the wiring path, and that not only is a space for installation and accommodation required, but it can be a cause of imposing limitations to the degree of freedom in the design of the wiring path. In addition, there is a problem of an increase in the coupling loss due to the insertion of the optical fiber.